Subwoofer boom is too much for me...

Real curious about what sub you have.

Years ago, my first sub, was nothing but boom. It was a low price Yamaha. 

Next sub was a Definitive Technology prosub 1000. It was better, but still had a little boom to it. In hindsight, I figure it was intended to serve in HT.

Got tired of playing that game and went to a REL 10" sub and found nirvana. Properly set up, a good sub is hard to notice, except for a little more depth and better definition of low frequencies.

@kota1  +1

I would think that a port needs to be plugged with something that seals it, like the foam plugs that come with some subs.  Maybe cut a piece off of a solid foam pool noodle.

Tony1954 above has the answer. I would do the same. But I already did.

Stuffing the port of a sub will make it less of a sub. The lowest notes of a ported box come from the port, not the driver. And if the box was not designed to be used stuffed as well as a higher bass cut off you could get boomy bass even from a 'sealed' enclosure if the new Q of the system is high.

Plus good ported bass sounds different from good sealed box bass. It tends to be richer due to more overhang than a good sealed box, A sealed box will tend to be more detailed but drier sounding.

I happen to have big bass boxes with 18" woofers designed to be used both sealed and ported because the designer was afraid of too much bass in some rooms. I discussed what I heard as described above to him and we totally agreed on what we heard. But I ended up with a closed box(with 10 Hz less bass extention). And even though he knew the closed box was more accurate he used the ported format.